The Development of the AlHgator 257 



it be here so called, is proportionately more shallow 

 than in the preceding figure and is actually much 

 wider. The section passes behind the posterior 

 end of the notochord, so that structure is not seen. 

 Though not so well indicated as might be desired 

 in the figure, the three germ layers are here indis- 

 tinguishable in the middle line, and in the center 

 of this mass of cells the blastopore (hip) or neu- 

 rentcric canal may be seen as a small vertical slit. 

 As will be more fully described in the following 

 stage, this canal opens dorsally a few sections 

 posterior to the one under discussion and ventrally 

 a few sections farther toward the head. 



In all the sections of this stage the ectoderm 

 and entoderm are fairly thick in the region of the 

 embryo proper, but become thinner until reduced 

 to a mere membrane as we pass to more distal 

 regions. Both layers are composed of loosely 

 arranged cells, with scattered nuclei. Where the 

 ectoderm becomes thickened to form the medul- 

 lary folds, the cells are much more compactly 

 arranged; hence this region stands out in strong 

 contrast to the rest of the ectoderm. 



STAGE VI 

 Figures ga-gm (Plates XIV., XV.) 



The embryo represented by this series of trans- 

 verse sections is intermediate in development 

 between those represented in surface views by 

 17 



